Radar systems are used to detect the presence of objects and to measure the location and movement of objects. In general, radar systems are designed for a specific application: to measure distance over a specified range of distances; over a specified scan region; within a specified level of accuracy; and in relation to a specified orientation. So-called crawler radar systems are intended for detection of targets that deliberately attempt to avoid detection by keeping low and by moving slowly. Such targets can be characterised as having a radar cross sectional area of approximately 0.1 m2 and moving with a speed of below 3 km/h, typically 1 km/h. Traditional radar systems that are adapted to provide crawler detection operate so as to measure thousands of small sections of land, typically 3° wide and 1 metre deep; measurements from successive scans of a given section are compared with one another, and a crawler moving into or out of a specific range cell at a particular bearing can be detected from a change in reflected energy between scans. If the land over which the crawler is moving is flat, then there is little radar energy reflected back to the radar from the land, enabling the crawler to be discriminated from the lower level background power. However, as soon as there is some level of “clutter” in the form of grass, bushes, trees etc. then the radar sees a considerably larger background clutter return, which makes it difficult, if not impossible, to distinguish the crawler from the clutter. For short grass this could be the equivalent of say one tenth of the area being illuminated by the radar: i.e. 1/10 of 5 m2 at 100 m (i.e. 0.5 m2). Even at this short range, the traditional crawler radar will struggle to detect the additional energy of a crawler of 0.1 m2 on top of the 0.5 m2 from the grass. At 1000 m, the comparison is 0.1 m2 on top of 5 m2, which is practically impossible to detect. As a result, traditional crawler radar systems are inherently limited to maximum detection ranges of only a few hundred meters.
Typically, conventional crawler radar systems do not use the Doppler characteristics of the targets as part of the detection criteria. This is partly because the targets move slowly, but also because certain of the key characteristics associated with a crawler target make it very difficult to utilise Doppler radar systems. Moreover conventional crawler radar systems use mechanically steered antennas, for which problems with Doppler processing, such as spectral widening, are particularly acute, as will now be explained. As a mechanically steered beam moves over the terrain, the transmitted power falling on any particular spot rises as the beam approaches and falls as it recedes. Consequently, for successive radar pulses, the return power for a given reflecting surface is modulated, resulting in a widening in frequency of the return from perfectly still clutter (e.g. the ground, buildings and foliage); the portions attributable to widening are referred to as “skirts”. As described in page 159 of “Radar Handbook” Published by McGraw-Hill (second edition), 1990, ISBN 0-07-057913-X, the standard deviation of this spread can be expressed as:
                    0.265        ×                              P            ⁢                                                  ⁢            R            ⁢                                                  ⁢            F                    n                ⁢                                  ⁢        Hz                            Equation        ⁢                                  ⁢                  (          1          )                    
where PRF is the pulse repetition frequency and n is the number of pulses generated while the antenna scans through the radar's 3 dB beam width.
                              Since          ⁢                                          ⁢          n                =                              P            ⁢                                                  ⁢            R            ⁢                                                  ⁢            F            ⁢                                                  ⁢                          (              Hz              )                        ×            3            ⁢                                                  ⁢                          dB              ·              beamwidth              ·                              (                °                )                                                          scanrate            ·                          (                              °                /                sec                            )                                                          Equation        ⁢                                  ⁢                  (          1          )                    can alternatively be expressed as
  0.265  ×            scanrate      ·              (                  °          /          sec                )                    3      ⁢                          ⁢      dB      ⁢                          ⁢              b        ·        beamwidth        ·                  (          °          )                      ⁢          ⁢  Hz
A scan rate of 35°/sec with a 3° beamwidth gives a standard deviation of 3.1 Hz. Typically 95% of the power of a Normal Distribution is contained in ±2 standard deviations, which implies that the static clutter energy could be expected to significantly affect the Doppler region extending approximately 6 Hz either side of the DC component associated with static clutter.
If the clutter comprises foliage, this will inevitably have a dynamic and weather-dependent characteristic, resulting in a further spectral widening of the return signals.
Turning to aspects of the signal processing, the spectral computation process assumes that the captured signal is one of an infinite number of identical sections, each abutting a successive section; however, the boundary between successive sections can include abrupt discontinuities. These are suppressed by means of windowing the signal between the successive sections, effectively importing returns from adjacent bins into a given bin. Whilst this has the benefit of reducing the effect of the discontinuities it also results in spectral spreading of each signal component, including the DC component associated with static clutter.